In recent years, for the consideration of the environment, hybrid vehicles (hereinafter referred to as “HEVs”) that are driven by an engine and a motor and electric vehicles (hereinafter referred to as “EVs”) that are driven by a motor only are widespread.
These HEVs and EVs have a power conversion device mounted thereon that converts DC power from a secondary battery into AC power to drive a motor during driving of vehicles and converts AC power regenerated from braking energy by the motor into DC power to charge the secondary battery during deceleration. The power conversion device includes a power module having a power semiconductor device and DC power and AC power are converted according to an ON/OFF operation of the power semiconductor device. Since the power conversion device mounted on HEVs and EVs is mounted in a limited mounting space, there is a demand for reducing the size and improving the efficiency.
Thus, it is effective to improve the cooling efficiency of the power module, and PTL 1 discloses a power module in which heat generated by a power semiconductor device is dissipated from both surfaces of the power module to reduce thermal resistance between the power semiconductor device and refrigerant that cools the power semiconductor device.
In such a power module, since the power semiconductor device needs to be electrically isolated from a housing (heat dissipating casing) of the power module storing the power semiconductor device, an insulating layer is formed between the power semiconductor device and the heat dissipating casing.
Since the power module repeatedly enters into a high-temperature state due to the heat generated by the power semiconductor device in operation and a low-temperature state in the stop state, the insulating layer is likely to be separated. When the insulating layer is separated, a discharge is more likely to occur through the gap formed by the separation. Although the insulating layer needs to be sufficiently thick in order to secure insulating properties even when the insulating layer is separated, the thicker the insulating layer, the higher the thermal resistance becomes.
The required thickness of the insulating layer needs to be determined by taking bubbles inside the insulating layer, the withstanding voltage when separated as above, and the properties and insulating properties of the insulating layer under various environmental conditions (temperature, humidity, atmospheric pressure, and the like) as well as the permittivity and withstanding voltage of the insulating layer into consideration. Thus, conventionally, the insulating layer thickness is determined by conducting a number of heat load cycle tests and dielectric withstand tests on experimentally manufactured power modules for a long period.